Federal prosecutors have issued grand jury subpoenas to associates of former National Security Advisor Michael Flynn seeking business records, as part of the ongoing probe of Russian meddling in last year's election, according to people familiar with the matter. CNN learned of the subpoenas hours before President Donald Trump fired FBI director James Comey.

The subpoenas represent the first sign of a significant escalation of activity in the FBI's broader investigation begun last July into possible ties between Trump campaign associates and Russia.

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The subpoenas issued in recent weeks by the US Attorney's Office in Alexandria, Virginia, were received by associates who worked with Flynn on contracts after he was forced out as director of the Defense Intelligence Agency in 2014, according to the people familiar with the investigation.

Robert Kelner, an attorney for Flynn, declined to comment. The US Attorney's Office in Alexandria, the Justice Department and the FBI also declined to comment.

Obama warned Trump about hiring Flynn

Investigators have been looking into possible wrongdoing in how Flynn handled disclosures about payments from clients tied to foreign governments including Russia and Turkey, US officials briefed on the matter have told CNN.

The Flynn inquiry is one piece of the broader investigation, which FBI Director James Comey testified in a Senate hearing last week is led jointly by the Alexandria US Attorney's Office and the Justice Department's National Security Division.

Flynn was forced to resign as Trump's national security advisor in February after failing to disclose the nature of phone discussions with Russia's ambassador in Washington, Sergey Kislyak.

Congressional investigators have also accused Flynn of possibly breaking the law by not properly disclosing a $45,000 payment for an appearance he made at an event in Moscow to celebrate Russia Today. The Russian government-funded news outlet that US intelligence agencies say played a key role in disseminating stolen emails intended to damage the candidacy of Hillary Clinton in the 2016 presidential election.

Flynn's lawyer, Robert Kelner, has said that Flynn was not hiding anything, noting that he briefed the DIA on his trip to Russia.

"As has previously been reported, General Flynn briefed the Defense Intelligence Agency, a component agency of DoD, extensively regarding the RT speaking event trip both before and after the trip, and he answered any questions that were posed by DIA concerning the trip during those briefings," Kelner said in a statement

In March, Flynn's lobbying firm registered as a foreign agent for the Turkish government, under a $500,000 contract.

Flynn, a retired Army lieutenant general, emerged in 2015 as a strident critic of the Obama administration, which fired him. He built a close relationship last year with then-candidate Trump and became a frequent Trump campaign surrogate before assuming a brief tenure as national security advisor.

US Attorney Dana Boente, whose office issued the subpoenas, is also leading the investigation into WikiLeaks and the effort to bring possible charges against the group's founder, Julian Assange. Boente is also acting as head of the Justice Department's national security section.

The FBI interviewed Flynn about the December calls with Kislyak and determined that he wasn't intentionally trying to be deceptive about the nature of what was discussed, according to US officials briefed on the investigation. But investigators have been investigating Flynn's business ties after he left the government and before he joined the Trump administration.